High pressure steam is introduced into steam turbine drives for dynamoelectric machines by way of controlling valves. Typically, a multitude of valves are mounted about such turbine drives. Such valves are equipped with actuators for positioning the valve so as to supply or extract steam at the pressures and quantities required under varying system conditions.
Modern control systems for turbine generators incorporate electronic speed and pressure sensors, along with digital processing and logic for, among other things, position control of steam turbine valves, as generally illustrated in FIG. 1, for example. Such valves are conventionally hydraulically operated by way of hydraulic servo-actuators within servo-actuator module 1, as determined by control signals as demanded from the operator control panel and display unit 2. The hydraulic servo-actuators in turn control the position of the steam valves by way of mechanical linkages.
Such hydraulic units conventionally include a remotely located hydraulic power unit 3, as well as tubing runs between the power unit and the valve actuators. These systems are relatively expensive since they require the use of either a phosphate ester hydraulic fluid including a Fuller's earth treatment system or, alternatively, petroleum based fluid with guarded hydraulic lines. Thus, leakage, contamination and possible low temperature problems may occur.
Still other known manners of controlling steam turbine valves involve pneumatic cylinders or diaphragms for relatively small control valves. In some older mechanically controlled turbine generator systems, the use of steam cylinders for operating low pressure grid-type extraction valves have also been known. However, the use of steam actuation is clearly not compatible with the requirements of modern electronic control systems of the nature generally illustrated in FIG. 1. Moreover, the use of pneumatic systems has the disadvantage of requiring large cylinders based on the use of available low pressure air and the resulting lack of system stiffness.
Still other more contemporary hydraulic systems involve a completely self-contained hydraulic system along with an actuator which are used for application to individual valves. It has been found, however, that such self-contained hydraulic systems for each of several valves in the contemplated environment would be applicable only to very large turbine units with off-shell steam chests. Moreover, such systems are relatively costly and require a significant increase in space with respect to other forms of actuators.
We have discovered that each of the steam turbine admission or extraction valves may have its own ball-screw linear electric actuator for operatively driving the valve open or closed by way of a lever, pins and links. Moreover, the actuator screw may be precisely driven by a DC brushless servomotor for accurately positioning the turbine valve at selected set points. Still further, since the ball-screw actuator arrangement is of a low-friction type, the actuators are fully capable of being driven to a closed position by way of an external force, such as that provided by a spring in the event of a power failure.
Accordingly, the objects of the herein disclosed exemplary embodiment include that of eliminating all hydraulics from the turbine control system including the conventional remotely located hydraulic power unit. Still further, all hydraulic tubing and fittings as well as mechanisms such as pinions, racks, cams and camshafts for these valves are eliminated. Thus, a more simple and less costly system is obtained whereby the use of hydraulic fluid and its attendant filtering, conditioning and leakage problems are absent. A still further object of the present system is that of employing a linear electric actuator for such steam valves which will require only electrical connections for handling position, feedback and power signals. In such a system the steam valves may be individually closed or opened in a relatively simple and flexible manner rather than being opened and closed in a fixed order, such as through the use of a camshaft.